Continuous wave ring laser gyros have evolved to a highly sophisticated state of development. Their proposed use in inertial guidance systems which stabilize airplanes, helicopters, weapons' delivery systems, cameras, radar antennas, and other vehicles is widespread. The gyros sense attitude changes about an axis of rotation and, usually three of them are orthogonally disposed with respect to one another to provide indications of three dimensional motion. One limitation of gaseous ring laser gyros is a phenomenon known as lock-in. This arises from coupling between the clockwise and counterclockwise modes of propagation and is attributed to the scattering of the gas molecules in the laser gain medium. The lock-in rate is limited to a value of about 0.003 degrees per hour.
Since the output of a laser gyro is observed as a function of the rotation rate, it can be seen that the difference frequency between the clockwise and counterclockwise modes is proportional to the input at high rates. However, as the input rate is reduced, the frequency difference between the two oscillators will fall to zero before the input rate goes to zero. The input rate at which this lock-in, zero difference frequency occurs is called the lock-in rate.
The lock-in rate is found to depend primarily upon the coupling factor which is dependent upon the wavelength. In order to reduce the lock-in rate, it has been found that the smaller the wavelength, the better, and the smaller the coupling factor, the better.
The choice of wavelengths is restricted to values at which laser oscillation can occur. The minimum limiting value of the coupling factor is determined by Rayleigh scattering which increases rapidly as the wavelength decreases but present gyros are far from this limitation due to imperfection gain medium backscattering.
Making the area enclosed by the optical pathway in the laser gyro falls within practical considerations of size, weight, and power. Usually, space limitations require that the gyro should be as small as possible, e.g., less than a meter in diameter.
The last consideration governing lock-in rate usually is determined by the quality of the laser cavity. Scattering from windows, mirrors, and other elements within the cavity should be reduced to an absolute minimum. However, there still exists the coupling that arises from the scattering of the gas molecules in the laser gain medium. As mentioned before, to date, coupling arising from scattering has prevented lock-in rate from being less than 0.003 degrees per hour in lasers operating in a continuous wave mode.
Thus, there is a continuing need in the state-of-the-art for a ring laser having a greatly reduced lock-in rate to provide a hitherto unrealized rotation rate resolution.